As those skilled in the art are aware, since the 1940-50's era, the search has been continuous in the quest for new alloys capable of withstanding increasingly severe operating conditions, notably temperature and stress, brought about by, inter alia, advanced designs. This has been evident, for example, in respect of gas turbine engine components such as combustors. Alloys of this type must be fabricable since they are often produced in complex shapes. But what is required apart from fabricability is a combination of properties, including good stress rupture life at high temperatures, 1600.degree.-2000.degree. F. (871.degree.-1093.degree. C.), low cycle fatigue, ductility, structural stability, high temperature corrosion resistance, and weldability.
In significant measure, alloys currently used for such applications are those of the solid-solution type in which there is substantial carbide hardening/strengthening but not much by way of precipitation hardening of, say, the Ni.sub.3 (Al, Ti) type (commonly referred to as gamma prime hardening). In the latter type the gamma prime precipitate tends to go back into solution circa 1700.degree.-1750.degree. F. (927.degree.-954.degree. C.) and thus is not available to impart strength at the higher temperatures. One of the most recognized and widely used solid-solution alloys is sold under the designation INCONEL.RTM. alloy 617, an alloy nominally containing 22% Cr, 12.5% Co, 9% Mo, 1.2% Al, 1.5% Fe with minor amounts of carbon and usually titanium. This alloy satisfies ASME Code cases 1956 (Sections 1 and 8 non-nuclear construction of plate, pipe and tube to 1650.degree. F.) and 1982 (Section 8 non-nuclear construction of pipe and tube to 1800.degree. F.).
Notwithstanding the many attributes of Alloy 617, as currently produced it has a stress rupture life of less than 20 hours, usually about 10 to 15 hours, under a stress of 11,000 psi (75.85 Mpa) and at a temperature of 1700.degree. F. (927.degree. C.). What is required is a strength level above 20 hours under such conditions. This would permit of the opportunity (a) to reduce weight at constant temperature, or (b) increase temperature at constant weight, or (c) both. In all cases gas turbine efficiency would be enhanced, provided other above mentioned properties were not adversely affected to any appreciable extent.
Perhaps a conventional approach might suggest increasing the grain size of an alloy such as 617 since the larger grain sizes, ASTM #1-#2, lend to stress-rupture strength. Alternatively, one might posit using a higher alloying content e.g., molybdenum, to achieve greater strength. But these approaches, depending on end use, may be limited or unavailable. For combustor sheet there are specifications which require about 4 to 10 grains across the gauge to thus ensure satisfactory ductility and adequate low cycle fatigue. This in turn would mean that the average grain size should not be much beyond ASTM #4 or #3. On the other hand, excessively high percentages of such constituents as molybdenum and chromium (matrix stiffeners) can result in the formation of deleterious amounts of subversive morphological phases such as sigma. This lends to embrittlement, phase instability and weldability and fabrication problems.